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Re: [Xen-devel] Design question for PV superpage support

To: "Mick.Jordan@xxxxxxx" <Mick.Jordan@xxxxxxx>, Dave McCracken <dcm@xxxxxxxx>
Subject: Re: [Xen-devel] Design question for PV superpage support
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 02 Mar 2009 17:06:21 +0000
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On 02/03/2009 16:43, "Mick Jordan" <Mick.Jordan@xxxxxxx> wrote:

> On 03/02/09 05:54, Dave McCracken wrote:
>> The solution I am working on for how to support Linux hugepages (Xen
>> superpages) involves creating domains made up entirely of superpages.  I can
>> create a working domain with superpages and am in the process of supporting
>> it in save/restore.
> This wouldn't work too well for me in the case of thread stacks because
> we need to map out parts of the stack and, although we want large
> virtual stacks, we don't want do dedicate that much physical memory. Is
> it really difficult to support mixed pages sizes in the general case,
> e.g., save/restore etc.?

You can still make 4kB mappings of subsections of 2MB physical extents. And
the guest kernel will still be able to allocate subsections of 2MB physical
extents for various uses. Isn't that all you need for e.g., this thread
stack situation?

Presumably Dave McCracken will be implementing a 'best effort' mode for
domains where we try to allocate superpages but we get by at reduced
performance if we have to allocate some discontiguous extents due to lack of
contiguous available memory. That would be reasonably sensible.

 -- Keir

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